Shift Data Points by

The "Shift Data Points by" option allows all the values for the independent variable to be shifted up by a fixed value. This is done by simply entering a value for "Shift Data Points by". For example, if your independent variable is years (input as 1975, 1976,...), but you would like these points to represented on the graph at the midpoint of the years (1975.5, 1976.5, ...), then you would enter the value 0.5 for this option. Shifting the data points will change the location of the joinpoints and the intercepts but will not change the slopes or APCs.

Note: this value cannot be greater than the maximum interval between data points.

One reason for employing this option would be because each data point represents a summary of data collected over a time interval. For example, cancer incidence or mortality data is often collected over the course of a year, and is usually entered as a whole year value, e.g. 1990, 1991. Instead, one may want to shift by half a year so that the data point is represented as the midpoint of the interval, e.g. 1990.5, 1991.5). This is especially important if joinpoints are allowed to occur at places other than the data points (either in continuous time using Hudson's algorithm, or using a grid search where grid points are allowed between data points). If the data points are not shifted, the results may be counterintuitive. For example, without a shift, a joinpoint located at 1989.50 represents the beginning of 1990, and 1990.49 represents the end of 1990. If the points are shifted by half a year, then 1990.00 represents the start of the year and 1990.99 represents the end of the year.